Need Help From the PROS!!!

I have 1997 Lesabre with 170k that has run perfect up until now. I have recently put new plugs and wires on the car along with a new air filter. Car has been 100% reliable to this point. Oh...no oil deposits on plugs and they are burning great. No check engine light and had the codes read at autozone with no codes stored.

Last week I discovered a coolant leak at the throttle body. I removed the throttle body and replaced the rubbber gasket which fixed the leak. Car ran great. I have been monitoring the coolant and it appears there are no internal leaks either.

Recently it has developed a miss or a stumble at idle that is pronounced when the car is in gear or the a/c is on. You can barely hear the miss in neutral but can feel the engine stumble every 3-5 seconds or so.

Any ideas right off the bat? What is the best way to diagnose such a general problem?

UPDATED

Ok...here is what I have done.
Checked plugs. All new plugs show great combustion, nice clean burn. Checked plug wires for fit and resistance. All are ok...and only 3 months old.
Replaced all coil packs with new ones with no change...also cleaned connectors with no change.
Tested fuel pressure. 50# with key on. 43# while running. 50-52 with vacuum line disconnested at pressure relay.
Ran engine in complete darkness with no obvious signs or arcing.

Car still has obvious miss or stumble that nearly stalls car when in drive, stopped and can feel miss during acceleration and driving.

Any more thoughts??

UPDATE #2

Buicks hate Bosch! Replaced my NEW Bosch Platinum plugs with AC Delco plugs and car runs fantastic...no more stumbles or misses. I couldn't find AC Delco wires tonight but will be replacing my new Bosch Platinum wires with the delcos asap. I didn't put much thought into the plugs being the problem as they are new with just a few thousand miles...had many bosch products in the past with no complaints. This is my first american, GM car so lesson learned...thanks to you all!!!

Any ideas right off the bat? What is the best way to diagnose such a general problem?
Just an idea off the bat. I've heard from different places if you replace the top intake gasket, the lower intake goes out soon after. I've found the dealer and a few other places won't put in just the upper gasket. They only will do both since, "your half way there".

Could be a leak in the lower intake manifold.
This is just a idea right off the bat. I can't say this is the problem. Just wanted to post anything I could help someone with. Chris.

I would first check for a loose connection at a plug or a bad wire and possibly a bad plug.Also double check all the bolts and connections you encountered during the gasket change.

In addition to the above, it could be something as simple as a small vacuum leak or a weak coil pack. Do you still have the original coils? If you had any vacuum lines off during the gasket replacement, do the old "carb cleaner" test of them. (Spray carb cleaner on the vacuum lines and see if the engine rpm increases).

Now, time for the more complex suggestions! :iceslolan

It could also be the first sign of a crank or cam sensor failure. When that first starts, it is erratic at first, & without a code. Have you ever replaced either of those? Also, with 170K on the engine, I would assume the timing chain is worn out, if it has never been replaced. It is a good idea to replace them before 150K. A worn timing chain will cause erratic idle and an overall loss of power when accelerating.

Back before we had all the sophisticated test equipment, we would put the car in gear, with the brake on, and bring the engine speed up to about 1500 rpm, to see if it has a miss under load. (I still use this method prior to hooking up all the testers & scanners). If it is missing under load, you may not feel it all the time, however it is still missing, you just don’t "feel it", due to the momentum & weight of the car. Additionally, this “load test” works very well at night when dark, so you can look under the hood for spark scatter around the ignition wires. If you have one jumping to ground, you may be able to see it!

:cya:

Updated original post....

http://www.websmileys.com/sm/comp/comp14.gif
Rather than throwing any more parts at it, I would hook it up to a "live scanner" and check all the sensor inputs & outputs.
In addition to the above, the EGR and O2 sensors can give the same symptoms.

Well, we are glad to here you got the car running nice again. If you had you mentioned that you had Bosch Platinum plugs in it earlier, you may have saved some time, expense and frustration. Like I have said, as well as many others in previous post, Buicks get sick on Bosch products! It has to do with the near “race car”, design of the DIS ignition and high voltage coils. Other cars can accept these products due to lower capacity ignition systems. This may have been an expensive lesson for you; however it is one you will never forget! Don't fell bad, we all go through this learning experience when we drive a Buick long enough to start doing routine maintenance!

By the way, most race car teams, use a non resistor, copper core plug, that have less resistance and capable of carrying high voltage. (Usually over 50,000 V). Platinum may last longer in a car that can accept them, however, the initial cost is double that of the regular plugs.
I guess it is a "break even" deal for the guys who use them, the "non Buick owners". :rofl: